Apparatus and methods for digital-to-analogue conversion

Abstract

In view of the limited speed times accuracy product of A-to-D and D-to-A converters, a required digital accuracy or sufficiently smooth analogue waveform cannot always be obtained. The present invention is useful in enhancing available converters. In the A-to-D version a stepped interpolation waveform obtained from a generator 13 is added to samples of an analogue waveform obtained from a sample-and-hold circuit 11 at a frequency 2f B (where f B is the bandwidth of the input signal). The resultant signal is applied to an n-bit A-to-D converter 15 operating at 2 m times 2f B and the converter output is averaged using an accumulator 16 reset at 2f B . The accumulator output is a higher accuracy signal since it has m+n bits. An analogous technique is described for D-to-A conversion.

I claim: 
     
       1. A digital-to-analog conversion device comprising in combination: (a) means for holding values of an n-bit digital signal to be converted one at a time, successive values of the n-bit digital signal being substituted at a first frequency;   (b) means for combining an output signal of the holding means with an interpolation signal that is periodic at a second frequency which is 2 m  times the first frequency, the combining means including an n-bit counter producing an n-m bit output signal, and means for setting the counter to the current value of the n-bit digital signal at the first frequency and means for incrementing or decrementing the counter at the second frequency during each period of the first frequency so that there are 2 m  times more values of the n-m bit output signal than of the n-bit digital signal; and   (c) an n-m bit digital-to-analog converter coupled to the n-m bit output signal and operating in response to a control signal that is periodic at the second frequency to convert each of the values of the n-m bit output signal to a corresponding analog value to produce an analog output signal having 2 m  analog values for each value of the n-bit digital signal.   
     
     
       2. The digital-to-analog conversion device of claim 1 including means for averaging the analog output signal. 
     
     
       3. The digital-to-analog conversion device of claim 2 wherein the analog output signal has a bandwidth f B , and the first frequency is at least 2f B  and m is a positive integer. 
     
     
       4. The digital-to-analog conversion device of claim 3 wherein the n-m most significant bits of the counter are coupled to the digital-to-analog converter. 
     
     
       5. The digital-to-analog conversion device of claim 4 wherein the averaging means includes a transversal filter receiving the analog output signal, wherein the transversal filter has unitary gain coefficients and 2 m  -1 delays each having a delay value of 1/(2 m  f B ). 
     
     
       6. The digital-to-analog conversion device of claim 4 wherein the averaging means includes a low-pass filter having a cut-off frequency of at least f B . 
     
     
       7. The digital-to-analog conversion device of claim 2 wherein the counter produces a carry signal, the digital-to-analog conversion device including logic gating means for coupling bit levels of the n-m bit output signal to n-m input terminals, respectively, of the n-m bit digital-to-analog converter, the logic gating means including n-m OR gates each having an input terminal receiving the carry signal and an input terminal receiving a respective bit level of the n-m bit output signal. 
     
     
       8. A method of converting a plurality of digital signals to an analog signal, the method comprising the steps of: (a) holding values of an n-bit digital signal to be converted one at a time in a holding means, and substituting successive values of the n-bit digital signal at a first frequency;   (b) combining an n-bit output signal of the holding means with an interpolation signal that is periodic at a second frequency which is 2 m  times the first frequency to produce an n-m bit output signal so that there are 2 m  times more values of the n-m bit output signal than of the n-bit digital signal by supplying the n-bit output signal of the holding means to the inputs of an n-bit counter, setting the n-bit counter to the current values of the n-bit digital signal at the first frequency, and incrementing or decrementing the n-bit counter at the second frequency; and   (c) coupling bit levels of the n-m bit output signal to n-m input terminals, respectively, of an n-m bit digital-to-analog converter and converting each of the values of the n-m bit output signal at the second frequency to 2 m  analog values at 2 m  equal intervals over the period of the first frequency.   
     
     
       9. The method of claim 8 including averaging the analog output signal produced by the n-m bit digital-to-analog converter. 
     
     
       10. The method of claim 9 wherein step (c) includes passing the n-m most significant bits of the counter to n-m input terminals, respectively, of the n-m bit digital-to-analog converter by means of n-m OR gates, one input terminal of each of the OR gates being connected to a carry output terminal of the n-bit counter.